Conversion of electromagnetic energy occurs when a change in flux is associated with mechanical motion as described by Faraday's law, e=d.PHI./dt. In alternators, voltages are typically generated in windings by rotating the windings mechanically through a magnetic field, by mechanically rotating a magnetic field past the windings or by designing the magnetic circuit such that the reluctance varies with the rotation of a rotor of the alternator. A time varying voltage is generated when the flux linking a specific coil is changed cyclically by any of the above-described methods.
One type of alternator is a brushless alternator. Brushless alternators can be viewed from a perspective of a variable reluctance transformer. To produce an AC output from the alternator, a varying magnetic field must be produced. The varying magnetic field may be accomplished using a permanent magnet for a magnetic core of the alternator and eliminating a primary direct current (DC) winding entirely, or by using a DC excitation. The permanent magnet approach can regulate the AC output only by varying the speed of the passage of the magnet with respect to the output winding. Thus, to produce rapid regulation responses to changing loads, large amounts of mechanical inertia must be overcome thereby resulting in a relatively sluggish regulation associated with large changing loads.
More specifically, a brushless alternator can be modeled as a variable transformer. Employing a DC excitation to produce a magnetic field, a smaller DC input power produces a larger output AC power. The additional power is the result of the mechanical force placed on the magnetic core. The output voltage increases with an increase of the motion of the magnetic core through the magnetic field (d.PHI./dt increases due to rate of travel), or when a stronger magnetic field is applied (d.PHI./dt increases due to stronger magnetic flux) to the magnetic core. To produce fast regulation of the AC output voltage at a constant speed, the DC current in the primary winding must be changed rapidly. The regulation speed is determined by a field driver, leakage inductances and magnetic remanences in the magnetic core.
The various alternator construction architectures presently employed include the homopolar, heteropolar, induction-type and brushless exciter. With a homopolar architecture, the magnetic flux is always in one direction. The flux intensity may vary, but the polarity (i.e., positive or negative) will remain the same. Since the flux is unidirectional and does not change polarity, only half of the total available flux change in the magnetic structure is used. As a result, the magnetic core saturates prematurely thereby placing a fixed upper limit on the total available output power. Limited flux reversal capability in a stator of the alternator is possible if a permanent magnet is used in the rotor, but regulation of the output voltage at no-load is still difficult. The speed of the rotor would have to decrease to zero to obtain d.PHI./dt=0.
The heteropolar architecture overcomes the saturation limitation associated with the homopolar model by allowing changes in the flux direction in the rotor as the rotor turns. The flux direction in the stator, however, is still unidirectional requiring twice as much magnetic material in the stator than if the flux could reverse. Furthermore, due to the phasing complexity of AC field excitation, a more complex measurement and control circuit is required.
For both the homopolar and heteropolar generators, the flux is always in the same direction on a "per-tooth" basis (i.e., each armature winding has only a modulated DC flux). As a result, only half (or less) of the magnetic material is used because of a DC bias in the armature of the alternator. Also, the output of the alternator may fold back as the magnetic field about the rotor increases.
The brushless exciter is used in many commercial AC generators currently in operation including large-scale commercial power generation. The exciter rotor and the alternator stator experience full flux reversal per rotation. The fluxes can be designed to maximize the use of the magnetic core material for specified power levels because the number of winding turns can be different on both the alternator and exciter rotors. The saturation limitation problem still exists because the flux is unidirectional in the alternator rotor. While rotary rectifiers are employed in the brushless excitor, the rectifier diodes experience voltage drops and heat losses that contribute to dissipative energy losses in the alternator. Furthermore, the rotary rectifiers must be mechanically secured to withstand the high centrifugal forces generated when the rotor is rotating. The stress on the rotary rectifiers decreases the reliability of the brushless exciter.
Full flux reversal can be obtained in both the stator and rotor by employing the induction-type alternator. A major limitation of the induction-type alternator is that a complex control circuit is necessary to provide a carefully phased AC for excitation thereof. Additionally, there is a need for shorted-turns windings (i.e., squirrel-cage windings) in the rotor thereby increasing dissipative heat losses in the alternator. In the inductor alternators (i.e., homopolar and heteropolar), as well as in the alternators used in automobiles (i.e., Lundell "claw" type), a magnetic field break is necessary due to the circulating parallel field in the shaft-core combination of the alternator. The magnetic field would otherwise pass through the bearings, generating arcs of DC electrical current, thereby causing premature deterioration of the bearings. The magnetic break in the shaft can produce a mechanically weak torsion point in the shaft that limits power from the prime mover.
Accordingly, there is a need in the art for an improved alternator design architecture that mitigates the above-mentioned limitations. In particular, there is a need for an improved alternator that maximizes the use of the magnetic flux to reduce the volume of magnetic core material required. There is a further need for an improved alternator that can respond to large load changes with faster speed regulation. Finally, there is a need for an improved alternator with fewer components and moving parts to increase the overall alternator reliability.